With the proliferation of digital devices, and in particular mobile devices, there is an increasing demand for high capacity, low power non-volatile memory. Currently, one of the most used types of non-volatile memory is flash memory. Flash memory is used in mobile smart phones, portable media players, video cameras, memory cards, Universal Serial Bus (USB) flash drives, gaming consoles, and the like. Further, flash memory has started to replace conventional hard disk drives, providing a solid-state alternative with low power consumption, greater reliability, and reduced size and weight particularly for mobile applications. High demand has led to rapid innovation in the design of flash memory devices, with high scalability resulting from the fact that each memory cell consists of only a single transistor and corresponding storage element buried inside the gate, or control, stack.
The conventional flash memory device includes a number of memory cells, where each memory cell is a floating gate transistor, comprised of a p-type Silicon substrate, a tunnel oxide, a highly doped n-type polysilicon storage layer (i.e., a floating gate), a control oxide, and a metallic electrode. Writing is achieved by applying a voltage pulse on the electrode, which allows electrons to tunnel through the tunnel oxide from the Silicon substrate into the polysilicon storage layer. This causes a positive shift in the threshold voltage of the Silicon substrate and is simply the additional voltage required to compensate the stored charge underneath the control oxide. The binary values are defined by the current upon a read cycle at a voltage within the width of the threshold voltage shift.
The primary goal of most research in flash memory is to scale down the size of the memory cells to thereby increase the density of memory cells, which enables increased storage capacity for a given die area or further miniaturization for a given amount of storage. However, the conventional flash memory device is now reaching its limit in terms of scalability and thus storage capacity. The primary issues that limit further scaling of the conventional flash memory device are Short Channel Effect (SCE), which is not limited to flash memory devices but universal to all metal-oxide-semiconductor based transistors, and cell to cell interference (i.e., interference between adjacent memory cells). Thus, there is a need for a flash memory device that has strong immunity to SCE and cell-to-cell interference and therefore can be scaled down beyond the limits of traditional flash memory devices.